1. Field of the Invention
The invention relates generally to the field of computer networking devices. More particularly, the invention relates to a method and apparatus for efficiently forwarding data, such as multicast data, from a source port to one or more destination ports of a networking device.
2. Description of the Related Art
Point-to-multipoint transmission is becoming increasingly important as the number of Internet and Local Area Network (LAN) applications that rely on multicast transmission continues to rise. As used herein "multicast" refers to the transmission of information to a group of recipients (e.g., a multicast group) via a single transmission by the source. Without multicast services, applications dependent upon such services would be forced to send information to groups of users with unicast packets. This means that a server, for instance, would have to send an individual copy of each data packet to all the users in an interest group (e.g., a Virtual LAN (VLAN), or other multicast group, such as an Internet Protocol (IP) multicast group). Unicasting to dispersed groups of users is inefficient for both the data source and the network, thus, network-based multicast services allow servers and other data sources to send out a single packet that is automatically copied and distributed by network devices, such as backbone routers or switches, to every endstation in the target group of users. Network devices that are configured to process multicast traffic typically maintain an internal mapping of multicast groups to member ports. In this manner, the network device can avoid unnecessary flooding of the data by forwarding multicast traffic only on those interfaces known to reach a member of the specified multicast group.
A prior approach for internally mapping multicast groups to interfaces having members is illustrated with reference to FIG. 1. A network device 100 includes a plurality of ports 105-108, switching logic 115, and a shared memory 110. The switching logic 115 maintains one or more output queues 111-113 in the shared memory 110 for each of the plurality of ports 105-108. The switching logic 115 determines with which output queues to associate received multicast data with reference to a multicast table 120. The multicast table 120 maps multicast group identifiers to corresponding per port bit vectors that identify members of the multicast groups. A processor (not shown) within the network device 100 may execute one or more well known multicast protocols, such as a group membership protocol (e.g., Internet Group Management Protocol (IGMP)) and a multicast routing protocol (e.g., Multicast Open Shortest Path First (MOSPF), Distance Vector Multicast Routing Protocol (DVMRP), Protocol Independent Multicasting (PIM)), to facilitate initialization and maintenance of the multicast table 120.
Because the switching logic 115 operates at a port level, a disadvantage of this prior approach is that the switching logic 115 must keep track of a substantial number of output queues in the shared memory 110. Additionally, the switching logic 115 interfaces with each of the ports 105-108. As a result, the switching logic 115 is complex and inefficient. Therefore, a more efficient mechanism for forwarding data through a networking device is needed.